Plastid Membrane Lipids in Stress and Development

The polar acyl lipids which provide the structural basis of all plant cell membranes all contain fatty acids synthesized in the plastid stroma. Fatty acid synthesis in the plastid stroma is also the source of acyl groups used for the synthesis of storage lipids in seeds and other tissues. While the basic biochemistry of fatty acid and glycerolipid synthesis is relatively well understood, the mechanisms for the transport of lipids between organelles and regulation of these pathways are just beginning to be unraveled. The plastid membranes themselves are composed of lipids synthesized in the plastid envelope. Notably, plastid membranes contain the non-phosphorous sugar-containing acyl lipids mono- (MGDG) and digalactosyl diacylglycerol (DGDG), and the sulfolipid, sulfoquinovosyl diacylglycerol (SQDG). These lipids are assembled in the plastid envelope from diacylglycerol backbones, synthesized by two different pathways: one contained entirely in the plastid envelope and one in the endoplasmic reticulum (ER). Phosphate limitation causes a dramatic remodeling of membrane lipids in the plant cell. In the thylakoid membrane, the anionic phospholipid phosphatidylglycerol (PG) is replaced by SQDG. In extraplastidial membranes (i.e. mitchondria, plasma membrane and the tonoplast) plastid envelopederived DGDG replaces several phospholipid classes. Plastidial lipids are also remodeled under other types of stress and in some cases storage lipids will accumulate inside plastids.